The Angels promoted Cowart to the major leagues on August 18, 2015.[5] He made his major league debut that night. After the 2015 season, Cowart changed his uniform number from 41 to 22. In 2016, he appeared in 31 games for the Angels, hitting .176. The following season, he played in 50 games, hitting .225 with 3 home runs. He began the 2018 season in AAA, being recalled during the season. In 47 games, he hit .134 with 1 home run and 10 RBIs.[citation needed]

On December 10, 2018, Cowart was claimed off waivers by the Seattle Mariners, who announced he would come to spring training as a third baseman and as a pitcher.[6] The Mariners designated him for assignment on January 21, 2019, and he was claimed off waivers by the Detroit Tigers on January 24.[7]